Methods for predicting kinase inhibitor resistance

ABSTRACT

Provided are methods for the identification of mutant kinases that are resistant to inhibition by a kinase inhibitor. In some embodiments, the methods may be used to assess a test compound or kinase inhibitor for the risk of the development of resistance in vivo, e.g., during clinical administration to treat a disease such as a cancer.

BACKGROUND OF THE INVENTION

This application is a divisional of U.S. application Ser. No.16/178,991, filed Nov. 2, 2018, which claims the benefit of U.S.Provisional Patent Application No. 62/580,556, filed Nov. 2, 2017, theentirety of each of which is incorporated herein by reference.

1. FIELD OF THE INVENTION

The present invention relates generally to the field of molecularbiology and medicine. More particularly, it concerns methods forassessing kinase inhibitors for the emergence of resistance mutations.

2. DESCRIPTION OF RELATED ART

Protein kinase inhibitors, including tyrosine kinase inhibitors, haveemerged as important therapeutic agents that may be used to treat avariety of diseases, such as various cancers. However, development ofresistance mutations in clinical populations that are administered thekinase inhibitor presents a significant clinical challenge.

Current methods for developing new protein kinase inhibitors aregenerally limited in their ability to predict the emergence ofresistance to the kinase inhibitor. Thus, while a kinase inhibitor mayyield clinical benefits for the treatment of a disease, such as acancer, these benefits may be quickly diminished or precluded if thecancer develops a mutation that allows the protein kinase to continue tofunction in the presence of the kinase inhibitor. Instances of theemergence of resistance to kinase inhibitors have been reported and areunderstood to be a major clinical problem (e.g., Levy et al., 2016).Clearly there is a need for both improved methods for predicting theemergence of resistance to a kinase inhibitor, as well as methods foridentifying kinase inhibitors that are less likely to result inresistant populations in vivo.

SUMMARY OF THE INVENTION

The present invention overcomes limitations in the prior art byproviding new methodologies to test for the possible emergence ofresistance to kinase inhibitors. In some aspects, YESS methodologies areused to evolve kinases in the presence of a kinase inhibitor or testcompound, and any mutant kinases that allow for continued kinaseactivity in the presence of the kinase inhibitor are subsequentlysequenced. In this way, it may be possible to evaluate the likelihood ofthe emergence of resistance in clinical populations in vivo. Forexample, in some embodiments, the emergence of mutant kinases thatinclude only one point mutation that allows for resistance to a kinaseinhibitor indicates a significant risk for the emergence of resistanceto the kinase inhibitor in clinical populations in vivo. In contrast, iftwo, three, or more point mutations are required in a mutant kinase toallow it to function in the presence of a kinase inhibitor, then therisk of emergence of resistance against the kinase inhibitor may bereduced. In some embodiments, methods of the present invention may beused to predict the risk of resistance to a test compound (e.g., akinase inhibitor) prior to administration of the test compound topatients to treat a disease (e.g., cancer). In some embodiments, alibrary of mutant kinases is generated using low-frequency mutations orPCR errors, resulting in a library of kinases containing each of all ofthe possible single point mutations in the kinase, and this library ofkinases may be tested for resistance to a kinase inhibitor. In someembodiments, the methods can be used to compare the risk of resistanceto a first kinase inhibitor (e.g., a test compound) to a second kinaseinhibitor (e.g., a clinically approved kinase inhibitor). As shown inthe below examples, these approaches were used to identify mutations inkinases that provide resistance against clinically approved kinaseinhibitors, and several of the identified resistance mutations have alsobeen observed in clinical patient populations in vivo.

In some embodiments, sequential exposure to a first kinase inhibitorfollowed by exposure to a second kinase inhibitor can be assessed usingthe methods disclosed herein. Thus, in some embodiments, one may assesswhich ordering of subsequent administration of different kinases mayyield the most clinical benefit (e.g., which kinase inhibitor should beadministered first, second, and so on in order to try to minimize thechances of development of kinase resistance in the patient). Forexample, in some embodiments, one may perform a selection using methodsdisclosed herein to produce a library of mutant kinases that areresistant to a first kinase inhibitor; subsequently, one may take thelibrary of kinase mutants and screen those mutant kinases against asecond kinase inhibitor (e.g., to see if any of the mutant kinasesdisplay or can easily develop resistance to the second kinaseinhibitor). This sequential screening can mimic what happens in theclinic when a patient is treated with one kinase inhibitor drug, becomesresistant to the drug, and then is treated with a different kinaseinhibitor drug. Further, this approach may allow one to determine whichsequential administration may yield the best therapeutic results. Forexample, if several or many of the mutant kinases resistant to a firstkinase inhibitor are often also resistant to a second kinase inhibitor,but the few or none of the mutant kinases resistant to the second kinaseinhibitor are resistant to the first kinase inhibitor, then it may beclinically advantageous to administer the second kinase inhibitor beforethe first kinase inhibitor. When two different kinase inhibitors aresequentially tested for emergence of resistance, the order of the kinaseinhibitors may be varied. Similarly, this approach may be used to testfor the emergence of resistance to a kinase inhibitor based on thesequential screening of three or more kinase inhibitors.

An aspect of the present invention relates to a method of generating amutant kinase, comprising: (a) expressing in each of a plurality ofeukaryotic cells: (i) a first fusion protein comprising an ER targetingsequence, a kinase, and an ER retention sequence; and (ii) a vectorencoding a second fusion protein comprising: an endoplasmic reticulum(ER) targeting sequence, a surface expression sequence, a first peptidesequence, and a endoplasmic reticulum (ER) retention sequence; whereinthe eukaryotic cells are exposed to a kinase inhibitor during saidexpression; and (b) separating or purifying said eukaryotic cells basedon the presence or absence of phosphorylation of at least one amino acidof the first peptide on the surface of the eukaryotic cells. In someembodiments, the method further comprises (c) wherein if at least someeukaryotic cells have said presence of phosphorylation of at least oneamino acid, then the method further comprises sequencing kinasesexpressed by the eukaryotic cells having said presence ofphosphorylation of at least one amino acid of the first peptide. Thekinase may be at least partially randomized. In some embodiments, thekinase is resistant to inhibition by the kinase inhibitor. In someembodiments, the kinase is a tyrosine kinase or a Src kinase. Theeukaryotic cell may be a yeast such as, e.g., a Kex2 (−/−) knockoutyeast. In some embodiments, the yeast has one, two, or all of the SNQ2,YOR1, and/or PDQ5 genes knocked out. SNQ2, YOR1, and PDQ5 can functionas small molecule transporters, and by knocking out one or more of thesegenes (e.g., in a yeast) it may be possible to reduce the export of akinase inhibitor from the cell. In some embodiments, the enzyme is akinase, and wherein the vector encodes a second fusion protein comprisesin an N− to C− direction: an endoplasmic reticulum (ER) targetingsequence, a surface expression sequence, the first peptide sequence, andan endoplasmic reticulum (ER) retention sequence. The sequencing maycomprise next-generation sequencing. The next-generation sequencing maycomprise or consist of single-molecule real-time sequencing, an ionsemiconductor method, a pyrosequencing method, a sequencing by synthesismethod, or a sequencing by ligation method. The method may furthercomprise analyzing data from said sequencing with a computer. In someembodiments, the endoplasmic reticulum (ER) targeting sequence encodedin the vector is comprised in said surface expression sequence in thevector. In some embodiments, the surface expression sequence is Aga2.The method may further comprise repeating steps (a) and (b). Theseparating may comprise or consist of fluorescence-activated cellsorting (FACS). The method may comprise repeated FACS separation andculture of the eukaryotic cells. In some embodiments, the enzyme is akinase and wherein step (b) comprises FACS separation of cells via anantibody that selectively binds a phosphorylated amino acid. Thephosphorylated amino acid may be a tyrosine. In some embodiments, thekinase is a human kinase (e.g., a tyrosine kinase). In some embodiments,the tyrosine kinase is a receptor tyrosine kinase or a non-receptortyrosine kinase. The non-receptor tyrosine kinase may be a Src kinase(e.g., c-SRC, YES1, Fyn, Fgr, Lck, HCK, BTK, Blk, Lyn, or Frk). In someembodiments, the kinase is ABL kinase, c-SRC, Lyn, or BTK. In someembodiments, a first promoter controls expression of the first fusionprotein, wherein the first promoter is expressable in yeast. The firstpromoter may be Gal1, Gal10, or Gal4-BS2-pleum. In some embodiments, theendoplasmic reticulum (ER) targeting sequence is MQLLRCFSIFSVIASVLA (SEQID NO:3). In some embodiments, the endoplasmic reticulum (ER) retentionsequence is FEHDEL (SEQ ID NO:4), KDEL (SEQ ID NO:5), HDEL (SEQ IDNO:6), or RDEL (SEQ ID NO:7). The purifying or separating may compriseseparating the cells based on the presence or absence of a firstantibody that selectively binds a phosphorylated amino acid (e.g., atyrosine). The antibody may be labeled with a fluorophore. The purifyingor separating may comprise or consist of fluorescence activated cellsorting (FACS). The method may further comprise an in vitro method forevaluating the risk of resistance to the kinase inhibitor in vivo.

In some embodiments, multiple resistant mutant kinases are generated andsequenced. The multiple mutant kinases are generated by error prone PCR(e.g., low-frequency error prone PCR). The multiple mutant kinases maybe generated by site directed mutagenesis. In some embodiments, themultiple mutant kinases are obtained from a library. In someembodiments, at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 single mutationsare observed to provide resistance to the kinase inhibitor. In someembodiments, at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 double mutationsare observed to provide resistance to the kinase inhibitor. In someembodiments, at least one of the multiple resistant kinases comprises amutation that has been observed in a subject that is resistant to thekinase inhibitor in vivo. The mutations in the resistant mutant kinasesmay be compared to a listing of mutations that can result in resistanceto a second kinase inhibitor such as, e.g., a kinase inhibitor has beenapproved for clinical use in vivo. In some embodiments, the listing isgenerated using a method of the present disclosure or as describedabove. In some embodiments, the listing is obtained by sequencingkinases obtained from patients resistant to the second kinase inhibitor.In some embodiments, the eukaryotic cell is a yeast, wherein the yeasthas one or more transporter genes knocked out. For example, in someembodiments, the yeast is a null mutant for 1, 2, or all of SNQ2, YOR1,and/or PDR5.

In some embodiments, the YESS method for measuring the activity of akinase in in a eukaryotic cell such as a yeast, comprises: (a)expressing in each of a plurality of eukaryotic cells: (i) a firstfusion protein comprising an ER targeting sequence, a kinase, and an ERretention sequence; and (ii) the vector encodes a second fusion proteincomprising: an endoplasmic reticulum (ER) targeting sequence, a surfaceexpression sequence, a first peptide sequence, and a endoplasmicreticulum (ER) retention sequence; wherein the eukaryotic cells areexposed to a kinase inhibitor during said expression; (b) separating orpurifying said eukaryotic cells based on the presence or absence ofphosphorylation of at least one amino acid of the first peptide on thesurface of the eukaryotic cells; and (c) wherein if at least someeukaryotic cells have said presence of phosphorylation of at least oneamino acid, then the method preferably further comprises sequencingkinases expressed by the eukaryotic cells having said presence ofphosphorylation of at least one amino acid of the first peptide. In someembodiments, the vector encodes the second fusion protein in an N− to C−direction: an endoplasmic reticulum (ER) targeting sequence, a surfaceexpression sequence, the first peptide sequence, and an endoplasmicreticulum (ER) retention sequence. In some embodiments, the vectorencodes the second fusion protein in an N− to C− direction: anendoplasmic reticulum (ER) targeting sequence, the first peptidesequence, a surface expression sequence, and a endoplasmic reticulum(ER) retention sequence. In some embodiments, it may be possible toexclude the surface expression sequence from the second vector, ifdesired. In some preferred embodiments, the eukaryotic cell is a yeastcell (e.g., a Kex2 knockout yeast cell).

In some embodiments, said sequencing comprises next-generationsequencing. In some preferred embodiments, the next-generationsequencing system is capable of longer reads that allow one to identifymultiple mutations in a single kinase, e.g., a Pacific Biosciencessequencing system. The next-generation sequencing may comprisesingle-molecule real-time sequencing, an ion semiconductor method, apyrosequencing method, a sequencing by synthesis method, or a sequencingby ligation method. The method may further comprise analyzing data fromsaid sequencing with a computer. For example, said analyzing maycomprises excluding sequences comprising a stop codon. In someembodiments, the endoplasmic reticulum (ER) targeting sequence encodedin the vector is comprised in the surface expression sequence in thevector. The surface expression sequence may be Aga2. In someembodiments, step (b) comprises repeated separations or multiple roundsof separation. In some embodiments, step (b) comprises multiple roundsof FACS separation and expansion or culture of the eukaryotic cells. Themethod may further comprise repeating steps (a) and (b). In someembodiments, the method comprises repeated FACS separation and cultureof the eukaryotic cells. In some embodiments, the first peptide may beless than 20 amino acids in length, less than 10 amino acids in length,or 4, 5, 6, 7, or 8 amino acids in length. The first peptide may becomprised in a protein, wherein the protein is encoded by the vector;however, in some preferred embodiments, the first peptide is notcomprised in a protein. In some embodiments, the first peptide comprisesone or more tyrosine residues that is flanked upstream and downstream bymultiple alanine residues. In some embodiments, said separatingcomprises fluorescence-activated cell sorting (FACS). In someembodiments, step (b) comprises FACS separation of cells via an antibodythat selectively binds a phosphorylated amino acid (e.g., aphosphorylated tyrosine). In some embodiments, the kinase is a humankinase, such as a tyrosine kinase or a Src kinase (e.g., c-SRC, YES1,Fyn, Fgr, Lck, HCK, Blk, Lyn, or Frk). In some embodiments, the kinaseis a wild-type kinase. In some embodiments, the kinaseis mutatedrelative to wild-type. The kinase may be a mutated kinase, e.g.,comprising 1, 2, 3, 4, 5, 6, or more substitution mutations, additions,or deletions as compared to the native or wild-type kinase but otherwiseshares complete amino acid sequence with the native or wild-type kinase.In some embodiments, a first promoter controls expression of the firstfusion protein, wherein the first promoter is expressible in yeast. Thefirst promoter may be Gal1, Gal10, or Gal4-BS2-pleum. The endoplasmicreticulum (ER) targeting sequence may be MQLLRCFSIFSVIASVLA (SEQ IDNO:3). The endoplasmic reticulum (ER) retention sequence may be FEHDEL(SEQ ID NO:4), KDEL (SEQ ID NO:5), HDEL (SEQ ID NO:6), or RDEL (SEQ IDNO:7). YESS methodologies are that may be utilized in variousembodiments are described, e.g., in U.S. Pat. Nos. 8,945,855, 9,546,359,PCT/US15/55494, Li et al. (2017), and Yi et al. (2013), which areincorporated herein by reference in their entirety.

In some embodiments, the yeast has one or more transporter gene knockedout. For example, the yeast may have 1, 2, or all of the following genesknocked out: SNQ2, YOR1, and PDR5; in some embodiments, the yeast is atriple-knockout where all of SNQ2, YOR1, and PDR5 have been knocked out.Since transporters can export some drugs from yeast (e.g., Watanabe etal., 2000; Kolaczkowska et al., 2008), by knocking out the transporterin yeast it is anticipated that this approach may be used to increaseinteractions between a drug or test compound with one or more kinases inthe cell. In some embodiments, it is anticipated that homologoustransporter genes may be similarly knocked out in other eukaryoticcells, if desired.

The term “antibody” is used herein in the broadest sense andspecifically encompasses at least monoclonal antibodies, polyclonalantibodies, multi-specific antibodies (e.g., bispecific antibodies),naturally polyspecific antibodies, chimeric antibodies, humanizedantibodies, human antibodies, and antibody fragments. An antibody is aprotein comprising one or more polypeptides substantially or partiallyencoded by immunoglobulin genes or fragments of immunoglobulin genes.The recognized immunoglobulin genes include the kappa, lambda, alpha,gamma, delta, epsilon, and mu constant region genes, as well as myriadimmunoglobulin variable region genes.

“Antibody fragments” comprise a portion of an intact antibody, forexample, one or more portions of the antigen-binding region thereof.Examples of antibody fragments include Fab, Fab′, F(ab′)2, and Fvfragments, diabodies, linear antibodies, single-chain antibodies, andmulti-specific antibodies formed from intact antibodies and antibodyfragments.

An “intact antibody” is one comprising full-length heavy- andlight-chains and an Fc region. An intact antibody is also referred to asa “full-length, heterodimeric” antibody or immunoglobulin.

The term “variable” refers to the portions of the immunoglobulin domainsthat exhibit variability in their sequence and that are involved indetermining the specificity and binding affinity of a particularantibody.

As used herein, the term “complementary nucleotide sequence” refers to asequence of nucleotides in a single-stranded molecule of DNA or RNA thatis sufficiently complementary to that on another single strand tospecifically hybridize to it with consequent hydrogen bonding.

An “expression vector” is intended to be any nucleotide molecule used totransport genetic information.

As used herein the specification, “a” or “an” may mean one or more. Asused herein in the claim(s), when used in conjunction with the word“comprising”, the words “a” or “an” may mean one or more than one.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive, although the disclosure supports a definitionthat refers to only alternatives and “and/or.” As used herein “another”may mean at least a second or more.

Throughout this application, the term “about” is used to indicate that avalue includes the inherent variation of error for the device, themethod being employed to determine the value, or the variation thatexists among the study subjects.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1 : Schematic of an example of library construction, expression andseparation of mutant or resistant kinases, and sequencing of mutant orresistant kinases.

FIG. 2 : Inhibition of wild-type ABL kinase and T315I ABL kinase byDasatinib and Ponatinib.

FIG. 3 : Enrichment of inhibitor resistance clones by FACS.

FIG. 4 : Sequencing results. Underlined mutations indicate that themutation has also been observed in vivo in patient populations that havebecome resistant to the kinase inhibitor.

FIG. 5 : In vitro validation of selected mutants. Ponatanib-selectedmutants were cloned into full-length BCR-ABL retroviral vectors andintegrated into the Ba/F3 murine pro-B cell line. Two of foursingle-mutants and two of three double-mutants had significantly higherIC₅₀ values than compared to wild-type ABL.

FIG. 6 : Map of vector pCMYpLeumG4BS12_SH3_SH2_BTK.

FIG. 7 : Map of vector pESD_PLCγ2_reporter.

FIG. 8 : subsequent rounds (Native, R2, R3) of YESS screening forresistant BTK mutants. Increased numbers of resistant mutants wereidentified in subsequent rounds of screening.

FIG. 9 : BTK C481S expressing cells retained APC signal in the presenceof Ibrutinib and Acalabrutinib compared to DMSO only

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention overcomes limitations in the prior art byproviding, in some aspects, methods for assessing kinase inhibitors forthe emergence of resistance mutations. In some embodiments, yeastendoplasmic reticulum sequestration sequencing (YESS) is used to evolvekinases that exhibit resistance to a test compound or kinase inhibitor(e.g., a tyrosine kinase inhibitor, a Src kinase inhibitor). In someaspects, by evolving mutant kinases in the presence of a kinaseinhibitor, mutant kinases that display resistance to inhibition by thekinase inhibitor may be identified. For example, as shown in the belowexamples these methods have been utilized to identify resistancemutations in vitro that have also been observed in vivo. In someaspects, the likelihood of the emergence of resistance mutations inpatient populations that might receive the kinase inhibitor (e.g.,cancer patients, etc.) can be assessed by the results of the identifiedresistance mutations. For example, for a particular kinase inhibitor, ifmultiple instances of a single point mutation are observed to conferresistance to the kinase inhibitor, then a significant risk for thedevelopment of resistance against the kinase inhibitor may be expectedwhen the kinase inhibitor is administered to patient populations in theclinic over a period of time. Alternately, if only multiple pointmutations in a kinase (e.g., two, three, or more substitution mutations)are observed to confer resistance to the kinase inhibitor, then theseresults can indicate a decreased risk for the development of resistancethe kinase when administered clinically to patient populations. In someaspects, methods described herein may be utilized, e.g., to assist inthe process of selecting kinase inhibitors that may warrant furtherclinical testing or usage. Wild-type or mutant kinases may be screenedfor resistance against a kinase inhibitor using methods provided herein.In some embodiments, the subsequent screening of a library of kinaseswith different kinase inhibitors may be performed, e.g., to determinewhich order kinase inhibitors could be administered to a patient inorder to minimize the emergence of resistance and/or increase thetherapeutic response.

I. YESS Screening Methods

In some aspects, methods of screening a test compound or kinaseinhibitor for resistance mutations may involve evolving or expressingkinases in the presence of a kinase inhibitor using yeast endoplasmicreticulum (ER) sequestration screening (YESS) methods. YESSmethodologies are described, e.g., in U.S. Pat. Nos. 8,945,855,9,546,359, PCT/US15/55494, and Li et al. (2017) and Yi et al. (2013),which are incorporated herein by reference in their entirety. In someembodiments, 1, 2, 3, 4, or more rounds of YESS screening may beperformed to evolve a kinase in the presence of a test compound or akinase inhibitor, and then the resulting mutant kinases that areresistant to inhibition by the test compound or kinase inhibitor may besequenced (e.g., using next-generation DNA sequencing). In someembodiments, next-generation DNA sequencing methods such as OxfordNanopore Technologies (Nanopore) or Pacific Biosciences (PacBio®) may beused in order to obtain longer reads in order to allow foridentification of mutations over a wider portion of the kinase; forexample, average lengths of 700 bp, 10 kb and 15 kb and maximum lengthsof 1 kb, 10 kb and 15 kb using these methods (Escalona et al., 2016).

YESS methods used in various aspects of the present invention generallyinvolve expression of kinases in yeast, or other eukaryotic or mammaliancells, for high-throughput screening. YESS may be used to identifyevolved protein kinases that are resistant to inhibition by a kinaseinhibitor or a test compound, and yeast cells expressing kinases thatare resistant to a kinase inhibitor or test compound can be separated,e.g., using fluorescence activated cell sorting (FACS). For example, ifa kinase can phosphorylate a target sequence expressed on the surface ofyeast, then this phosphorylated amino acid may be bound by an antibodythat selectively binds to the phosphorylated sequence and is tagged witha fluorescent label, and then the yeast may be separated based onfluorescence of the bound antibody using FACS. YESS may involve thetargeted interaction of the protein kinase variant with a targetsequence in the yeast endoplasmic reticulum (ER), in the presence of akinase inhibitor. Following reaction with the kinase in the ER,substrate phosphorylation products are preferably directed to display onthe yeast surface and detected with labeled antibodies. Cells thatexpress a kinase that is resistant to the kinase inhibitor can beseparated (e.g., using FACS) and sequenced. For example, antibodies thatselectively bind a phosphorylated amino acid in an amino acid sequencemay be used to detect phosphorylation by a wild-type or mutant kinaseusing these methods.

In some preferred embodiments, the YESS platform is used in combinationwith NextGen sequencing and a comparative sequence analysis is performedto identify mutant kinases that are resistant to inhibition of activityin the presence of a kinase inhibitor or a test compound. Nonetheless,in some embodiments, a library of kinases may include wild-type kinasesand/or kinases which have been previously identified as resistant toinhibition or inhibition by kinase inhibitors. Generally, an Aga2-taggedsubstrate library (expressing (i) a kinase, such as a kinase with one ormore randomized portions or one or more kinases that have a mutationthat may lead to resistance to a kinase inhibitor, and (ii) a targetsequence comprising a sequence that may be phosphorylated by the kinase)is targeted to the yeast endoplasmic reticulum (ER) and transportedthrough the secretory pathway, where the kinase can interact with atarget sequence (encoding a sequence that can be phosphorylated by thekinase in the absence of a kinase inhibitor, such as by a correspondingwild-type kinase in the absence of a kinase inhibitor) in the ER.Preferably this interaction between the kinase and the target sequenceoccurs in the presence of a kinase inhibitor or a test compound. Afterbeing transported outside of the cell and attached to the yeast surface,the substrate/product can be probed with fluorescently labeledantibodies for the presence or absence of phosphorylation of the targetsequence. This process may be carried out in the presence or absence ofa kinase inhibitor or test compound. In some preferred embodiments, theyeast cells are incubated in the presence of a kinase inhibitor or atest compound, in order to select for mutant kinases that are resistantto inhibition by the kinase inhibitor or test compound. An expressiontag may also be expressed in a fusion construct with the targetsequence, and the expression tag may facilitate separation of cells thatexpress a phosphorylated target sequence. Multi-color FACS screening maythen be used to isolate cells with appropriately phosphorylatedsubstrate. This process may be repeated 1, 2, 3, 4, or more times, asdesired. Then, next generation DNA sequencing (NextGen) may be used todetermine the sequence of the kinases that are resistant to inhibitionby the kinase inhibitor or the test compound.

In some embodiments, the yeast cleaveOme identified by this method maybe used to prepare target sequences that may avoid degradation whentransported via the yeast secretory pathway (Yi et al., 2017). Suchembodiments may be particularly useful to address or avoid problemsassociated with proteolytic degradation of a recombinant protein in ayeast cell during production of the recombinant protein in yeast. Insome embodiments, Kex2 knockout yeast (e.g., EBY100^(Kex2)) are used toexpress the kinase and substrate sequence. Kex2 (also known as kexin,peptidase 3.4.21.61) exists in the yeast secretory pathway (Seidah etal. 2002), and use of Kex2 knockout yeast may facilitate expression ofkinases in the endoplasmic reticulum (ER), without undesired cleavage ofthe kinase or substrate sequence in the ER of the yeast.

Some aspects relate to detecting the kinase activity in a eukaryoticcell, such as a yeast (e.g., a Kex2 knockout yeast). A vector expressinga first fusion protein comprising a peptide sequence and cell surfaceexpression sequence may be expressed in the cell. Then, the presence orabsence of phosphorylation of an amino acid in the peptide may bedetected, e.g., using FACS, based on the presence or absence of thebinding of an antibody that selectively recognizes a phosphorylatedamino acid. As would be appreciated by one of skill in the art, severalantibodies that selectively recognize phosphorylated amino acids (e.g.,phosphor-tyrosine, etc.) are commercially available. The first fusionprotein may further comprise an ER targeting and ER retention sequence.A wild-type or engineered kinase (e.g., a kinase with a randomizedportion, or select kinases that may be resistant to inhibition by akinase inhibitor) may also be expressed in the cell, e.g., in the samevector as the first fusion protein or in a different vector. In someembodiments, a portion of the kinase is randomized. The kinase mayfurther comprise an ER targeting and ER retention sequence. In this way,when the kinase and the first fusion protein each comprise an ERtargeting and ER retention sequence, the kinase and first fusion proteinmay be brought into closer proximity in the ER and/or benefit from theimproved folding environment of the ER. The yeast may be incubated inthe presence of a kinase inhibitor or a test compound. In someembodiments, the activity of a kinase may be measured by expressing thekinase in eukaryotic cells with the first fusion protein, detecting theactivity of the kinases expressed by the yeast cells, separating thecells that exhibit kinase activity in the presence of the kinaseinhibitor or test compound, and subsequently sequencing the kinases thatare resistant to inhibition (e.g., via next-generation sequencing).

Another aspect of the present invention relates to a method of measuringthe activity or specificity of a kinase, comprising: (a) expressing a ina plurality of eukaryotic cells a vector encoding an endoplasmicreticulum (ER) targeting sequence and a endoplasmic reticulum (ER)retention sequence, a surface expression sequence and the first peptidesequence; (b) purifying or separating the cells based on the presence orabsence of a first antibody that selectively binds a phosphorylatedamino acid; (c) sequencing the first peptide sequences after step (b) toproduce a dataset; and (d) subtracting or eliminating endogenous kinaseactivity in the eukaryotic cells from the dataset. In some aspects, theendogenous kinase activity of a cell or yeast may be determined by amethod of the present invention. The cells may be yeast cells (e.g.,Kex2 knockout yeast cells). The antibody may be labeled with afluorophore. The purifying or separating may comprise or consists offluorescence activated cell sorting (FACS).

The method may further comprise expressing a kinase in the yeast and/orrandomizing one or more amino acids in the kinase. The method maycomprise further characterizing the kinase. The kinase may be a humankinase such as, e.g., a tyrosine kinase or a Src kinase. The kinase maybe a wild-type kinase. In some embodiments, the kinase is mutatedrelative to wild-type. In some embodiments, at least a portion of thekinase is randomized. In some embodiments, the method is further definedas a method of generating an engineered kinase, wherein step (b) isrepeated. In some embodiments, the first endoplasmic reticulum (ER)targeting sequence and the second endoplasmic reticulum (ER) targetingsequence are MQLLRCFSIFSVIASVLA (SEQ ID NO:3). In some embodiments, thefirst endoplasmic reticulum (ER) retention sequence and the secondendoplasmic reticulum (ER) retention sequence are FEHDEL (SEQ ID NO:4),KDEL (SEQ ID NO:5), HDEL (SEQ ID NO:6), or RDEL (SEQ ID NO:7). Thekinase may be a mutated kinase, e.g., comprising 1, 2, 3, 4, 5, 6, ormore substitution mutations, additions, or deletions as compared to thenative or wild-type kinase but otherwise shares complete amino acidsequence with the native or wild-type kinase. In some embodiments, afirst promoter controls expression of the first fusion protein, and asecond promoter controls expression of the second fusion protein. Thefirst promoter and the second promoter are preferably expressible inyeast. In some embodiments, the first promoter is Gal1, Gal10, orGal4-BS2-pleum. In some embodiments, the second promoter is Gal1, Gal10,or Gal4-BS2-pleum.

A variety of concentrations of the kinase inhibitor or test compound maybe used to incubate yeast during the expression of the kinase and thetarget sequence. In some embodiments, a kinase inhibitor is used at aconcentration that approximates or is similar to concentrations thatwould be used clinically or might be achieved in vivo. In otherembodiments, concentrations of the kinase inhibitor may be used whichare substantially above or below the concentrations of the kinaseinhibitor that would be used clinically or that may be achieved in vivo.For example, incubating yeast that express the kinase and the targetsequence with a kinase inhibitor present at concentrations higher thanmay be achieved in vivo may be used to identify kinase mutants thatexhibit increased resistance to inhibition by the kinase inhibitor. Insome embodiments, 0.1, 1, 5, 10, 15, 20, 25, 30, 40, 50 μM or more orany range derivable therein of a kinase inhibitor may be used. In someembodiments, the yeast are incubated in the presence of kinase inhibitoror test compound at a concentration that is similar to or within a rangethat could be used clinically or that might be achieved in vivo, andthen the observed mutant kinases can be sequenced to assess the risk ofthe kinase inhibitor or test compound to the development of resistance,e.g., if administered clinically (e.g., to cancer patients, etc.).Kinase inhibitors that may be tested with and/or used in various aspectsof the present invention include, e.g., afatinib, axitinib, bosutinib,cetuximab, cobimetinib, crizotinib, cabozantinib, dasatinib,entrectinib, erlotinib, fostamatinib, gefitinib, ibrutinib, imatinib,lapatinib, lenvatinib, mubritinib, nilotinib, pazopanib, pegaptanib,ponatinib, ruxolitinib, sorafenib, sunitinib, SU6656, vandetanib, and/orvemurafenib. In some embodiments, cells are incubated in the presence ofkinase inhibitor a concentration that is similar to or essentially thesame as a concentration of the kinase inhibitor that may be achieved invivo, to identify resistant mutant kinases.

In some embodiments, the methods described herein may be used toevaluate the risk of emergence of resistance to a test compound or afirst kinase inhibitor, and this data may be compared with data obtainedusing the methods on a second kinase inhibitor or an approved kinaseinhibitor. In some embodiments, YESS sequencing may be used to identifymutations that are capable of allowing a kinase to function in thepresence of a test kinase inhibitor, and by comparing the mutations thatare required to impart resistance to the test compound to resistancemutations observed for a clinically approved kinase inhibitor, one maybe able to predict if the development of resistance in vivo is morelikely or less likely for the test kinase inhibitor, as compared to theclinically approved kinase inhibitor. For example, if few or no singlemutations, and only some double or triple mutations are observed toprovide resistance to a kinase against a test kinase inhibitor, then thechances for the development of resistance in vivo would be less than fora second kinase inhibitor (e.g., an approved kinase inhibitor), wherethe second kinase inhibitor has more or many single mutations that canresult in resistance. Similarly, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ormore approved kinase inhibitors may be tested using the methodsdescribed herein to allow for the ranking of the kinase inhibitorsaccording to relative risk for the development of resistance in vivo.For example, as shown in the below examples, multiple single mutationswere observed to provide resistance against dasatinib, and fewer singlemutations and more double mutants were observed to provide resistanceagainst ponatinib; thus, the risk for the emergence of resistance invivo is greater for dasatinib than for ponatinib.

II. Kinase Inhibitors

Protein kinases are well known in the art, and aberrant or increasedactivity of these enzymes have been associated with a variety ofdiseases, including cancers. Protein kinases typically add a phosphate(PO₄) group to an amino acid in a protein, such as a tyrosine in thecase of tyrosine kinases. Kinase inhibitors, also called protein kinaseinhibitors, inhibit the activity of the protein kinase. In someembodiments, the kinase inhibitor inhibits a tyrosine kinase. In someembodiments, a kinase inhibitor may be tested via the methods disclosedherein in order to assess the risk or resistance prior to administrationto human subjects.

In some embodiments, kinases can be evolved (e.g., using YESS) in thepresence of a known or approved protein kinase inhibitor, to identifyresistance mutations. The resulting mutants that are observed fromevolution of kinases in the presence of the known or approved kinaseinhibitor may be useful, e.g., for a medical professional when decidingwhich kinase inhibitor to prescribe to a patient. In some embodiments,kinases are evolved in the presence of a test compound or test kinaseinhibitor (e.g., that is not yet clinically approved), and the resultingresistance mutations may be compared to the resistance mutationsobserved for a known or approved kinase inhibitor; in this way, themethods may be used to predict if the risk for the development ofresistance mutation(s) is greater than or less than the risk associatedwith the known or approved kinase inhibitor.

In some embodiments, kinases can be evolved (e.g., using YESS) in thepresence of two protein kinase inhibitors. In these embodiments,particular combinations of kinase inhibitors may be identified that maybe particularly useful for decreasing the chances of emergence ofresistance mutations in clinical populations. Thus, these approaches maybe used to identify combinations of kinase inhibitors that may beco-administered to a subject to reduce the chances of the emergence of

A variety of protein kinase inhibitors are known and may be used invarious embodiments of the present invention. For example, the proteinkinase inhibitor may be a tyrosine kinase inhibitor such as, e.g.,gefitinib, imatinib, dasatinib, nilotinib, bosutinib, or ponatinib. Insome embodiments, the kinase inhibitor is afatinib, axitinib, bosutinib,cetuximab, cobimetinib, crizotinib, cabozantinib, dasatinib,entrectinib, erlotinib, fostamatinib, gefitinib, ibrutinib, imatinib,lapatinib, lenvatinib, mubritinib, nilotinib, pazopanib, pegaptanib,ponatinib, ruxolitinib, sorafenib, sunitinib, SU6656, vandetanib, and/orvemurafenib. In some embodiments, the protein kinase inhibitor has notbeen clinically approved or is a test compound (e.g., a small molecule,an antibody, or an antibody fragment, etc.). In some embodiments, a testcompound may be tested to identify mutant kinase(s) that are resistantto inhibition by the test compound, and these results may optionally becompared to results obtained for mutant kinases that are resistant toinhibition by an approved kinase inhibitor; in this way, it may bepossible to evaluate whether or not the test compound may be more orless likely to result in resistant patient populations in vivo, ascompared to the approved kinase inhibitor.

In some embodiments, the kinase inhibitor or test compound may be usefulfor the treatment of a disease such as, e.g., a cancer. For example, insome embodiments, the cancer is a non-small cell lung cancer (NSCLC), alung cancer, chronic myeloid leukemia (CML), gastrointestinal stromaltumors (GIST), renal cell carcinoma, a melanoma, a breast cancer, athyroid cancer, a renal cancer, a soft tissue sarcoma, a leukemia,chronic lymphocytic leukemia (CLL), small lymphocytic lymphoma (SLL), aB-cell malignancy, a pancreatic cancer, colorectal cancer, a bonecancer, a gastrointestinal cancer, a brain cancer, a thyroid cancer, orother cancer. The cancer may be metastatic or non-metastatic. In someembodiments, the disease is inflammation or an autoimmune disease.

III. Kinases

A variety of kinases may be expressed and tested in various embodimentsof the present invention. For example, in some embodiments, the kinaseis a tyrosine kinase. Tyrosine kinases can catalytically covalentlyattach a phosphate group to the amino acid tyrosine on a protein.Tyrosine kinases are well known to affect signal transduction involvedin cellular activities such as cell division, and overexpression oftyrosine kinases in a cell can result in or promote the emergence ofcancers, including non-small cell lung cancer, chronic myeloid leukemia,and gastrointestinal stromal tumors.

In some embodiments, the tyrosine kinase is a receptor tyrosine kinaseor a non-receptor tyrosine kinase. For example, in some embodiments, thetyrosine kinase is a receptor tyrosine kinase such as a RTK class I (EGFreceptor family) (ErbB family), RTK class II (Insulin receptor family),RTK class III (PDGF receptor family), RTK class IV (VEGF receptorsfamily), RTK class V (FGF receptor family), RTK class VI (CCK receptorfamily), RTK class VII (NGF receptor family), RTK class VIII (HGFreceptor family), RTK class IX (Eph receptor family), RTK class X (AXLreceptor family), RTK class XI (TIE receptor family), RTK class XII (RYKreceptor family), RTK class XIII (DDR receptor family), RTK class XIV(RET receptor family), RTK class XV (ROS receptor family), RTK class XVI(LTK receptor family), RTK class XVII (ROR receptor family), RTK classXVIII (MuSK receptor family), RTK class XIX (LMR receptor), or a RTKclass XX tyrosine kinase. In some embodiments, in tyrosine kinase is nota receptor tyrosine kinase.

In some embodiments, the tyrosine kinase is a cytoplasmic ornon-receptor tyrosine kinase, such as a Src kinase or an Abl kinase. Insome embodiments, the tyrosine kinase is a Src kinase such as c-Src,Yes, Fyn, Fgr, Yrk, Lyn, Blk, Hck, or Lck. In some embodiments, thekinase is Bruton's tyrosine kinase (BTK). A variety of tyrosine kinasesare known and may be used in various embodiments (e.g., Espada et al.,2017; Varkaris et al., 2014.).

IV. Nucleic Acid Constructs

Certain aspects of the present invention involve nucleic acids thatencode an enzyme (e.g., kinase) that can modify a genetically encodedsubstrate and/or an amino acid substrate. The enzyme (e.g., kinase) andthe substrate may be expressed as a fusion protein with one or moreadditional sequences, such as an ER targeting sequence, an ER retentionsequence, a cell-surface sequence, and/or one or more immunotagsequences. In some embodiments, a single nucleic acid may be used toexpress both a kinase and an amino acid substrate in a cell. It isgenerally anticipated that, although expressing both a kinase and anamino acid substrate from a single vector or construct may effectivelyallow for interactions between the kinase and amino acid substrate in acell, in some embodiments the kinase and amino acid substrate may beencoded by two different or separate nucleic acids or vectors, and thetwo nucleic acids may be expressed in a cell, such as a yeast cell.

In some embodiments, the following construct may be generated and used.Under the control of the GAL10 promoter and after the Aga2 gene used foryeast surface display, a five-part cassette may be cloned that includes;(1) a first epitope tag sequence (e.g., a FLAG tag, DYKDDDDK, SEQ IDNO:9); (2) an amino acid sequence that can be phosphorylated by thewild-type kinase; (4) a second epitope tag (e.g., 6xHis tag, HHHHHH, SEQID NO:11); and (5) an ER retention signal peptide (e.g., FEHDEL, SEQ IDNO:4). Under the control of the GAL1 promoter, a kinase library (e.g., alibrary of mutant kinases, a library of kinases comprising one or morerandom mutations, a library of kinases comprising mutant kinasesresistant to inhibition by a kinase inhibitor) may be cloned along witha designed N-terminal ER targeting signal peptide (QLLRCFSIFSVIASVLA,SEQ ID NO:12) and with or without a C-terminal ER retention signalpeptide. If desired, the first epitope tag sequence and/or the secondepitope tag sequence may be excluded from the construct.

It is anticipated that a variety of target sequences may be used thatmay be phosphorylated by a kinase. Examples of target sequences that maybe used in various embodiments of the present invention include, but arenot limited to: AAAAAYAAAAA (SEQ ID NO:1), e.g., in embodiments usingAbl kinase.

Endoplasmic Reticulum (ER) Targeting Sequences

The construct may comprise 1, 2, or more sequences for targeting anamino acid sequence (e.g., comprising a kinase or a substrate sequence)to the endoplasmic reticulum (ER). In some embodiments, the HDEL (SEQ IDNO:6) system may be used as described in Monnat et al. (2000), which isincorporated by reference herein in its entirety. In some embodiments,the ER targeting signal peptide (QLLRCFSIFSVIASVLA, SEQ ID NO:12) isused. The ER targeting signal peptide may be at or near the N-terminalportion such that an amino acid comprising a kinase or substratesequence can be targeted to the ER.

Without wishing to be bound by any theory, the ER targeting sequence maybind a ribosome and allow for the amino acid to be transported into theER. Generally, an ER targeting sequence may promote entry of an aminoacid sequence, peptide, or protein, by promoting entry of the proteininto the ER through the translocon, e.g., via a protein-conductingchannel formed by a conserved, heterotrimeric membrane-protein complexreferred to as the Sec61 or SecY complex. In some embodiments, asequence disclosed as an ER targeting sequence of Rapoport (2007), Hedgeand Keenan (2011), or Park and Rapoport (2012) may be used with thepresent invention. In some embodiments, an N-terminal targeting sequencefor promoting entry into the endoplasmic reticulum may be identified viathe Predotar (Prediction of Organelle Targeting sequences) methoddisclosed in Small et al. (2004).

Endoplasmic Reticulum (ER) Retention Sequences

Once in the ER, in certain embodiments, it may be preferable to includean ER retention sequence or peptide in order to allow or promote anamino acid (e.g., comprising a kinase or a substrate sequence) to remainin the interior of the ER.

In some embodiments, the ER retention signal peptide is FEHDEL (SEQ IDNO:4). The HDEL (SEQ ID NO:6) system may be used as described in Monnatet al. (2000). In some embodiments, a protein chimera may be generatedthat contains a C-terminal tetrapeptide sequences of (−KDEL (SEQ IDNO:5), -HDEL (SEQ ID NO:6), or -RDEL (SEQ ID NO:7)) to promote retentionin the ER. If only a partial retention in the ER is desired, a proteinchimera may be generated that contains C-terminal sequence (−KEEL, SEQID NO:16). In some embodiments where it is desirable to use a mammaliancell line for expression of constructs, it may be useful to use themammalian (−KDEL, SEQ ID NO:5) sequence in a fusion protein with akinase and/or a substrate. The particular ER retention sequence used maybe chosen based on the amount of retention in the ER produced in aparticular eukaryotic cell type. In some embodiments, an upstreamsequence beyond the C-terminal tetrapeptide may be included that caninfluence or may be part of the structure of reticuloplasmin retentionsignals. In various aspects, a sequence may be included in a chimerickinase or in a chimeric substrate that promotes retention of the proteinor peptide in the ER by affecting sorting of exported protein, retentionof residents, and/or retrieval of escapees.

HDEL (SEQ ID NO:6) sequences are further described in Denecke et al.(1992). In some embodiments, an ER targeting sequence or ER retentionsequence of Copic et al. (2009) may be used. In some embodiments, anER-targeting sequence, such as the cytoplasmic KKXX (SEQ ID NO:17) or RRof Teasdale and Jackson (1996), may be used. The ER-targeting sequencemay be a Kar2p retention mutant, e.g., as described in Copic et al.(2009). In some embodiments, the C-terminal sequence-VEKPFAIAKE (SEQ IDNO:18) described in Arber et al. (1992), may be used to promotelocalization to a subcompartment of the ER. Each of the foregoingreferences is incorporated by reference in its entirety.

Epitope Tag Sequences

A construct of the present invention may comprise one, two, or moreepitope tag or immunotag sequences conjugated to or expressed as afusion protein with the substrate target on the surface of a cell (e.g.,a yeast cell). It is anticipated that virtually any epitope tag may beused in various embodiments of the present invention. For example,epitope tags that may be included in a peptide or encoded by a nucleicacid of the present invention include, e.g., FLAG, 6xHis, hemagglutinin(HA), HIS, c-Myc, VSV-G, V5 HSV, and any peptide sequence for which amonoclonal antibody is available. Antibodies that selectively bind theepitope tag sequences may be used to detect the presence or absence ofthe epitope tag(s); for example, a first antibody with a firstfluorophore may be used to detect the presence or absence of a firstepitope tag sequence, a second antibody with a second fluorophore may beused to detect the presence or absence of a second epitope tag sequence,and additional antibodies may be used to detect the presence or absenceof a third, or more epitope tag, as desired. In some embodiments, theantibodies are labeled with a dye, such as a fluorophore, and used forcell sorting. As would be appreciated by one of skill in the art, a widevariety of antibodies that selectively recognize an epitope tag and arelabeled with a detectable label such as a fluorophore are commerciallyavailable. Antibodies that selectively bind different epitope tags maybe labeled with different fluorophores; in this way, cells may beseparated or purified based on the presence or absence of one, two,three, or more fluorescent signals, e.g., using ratiometric FACS.

A wide variety of epitope tags have been engineered into recombinantproteins and may be used in various embodiments of the presentinvention. Epitope tags that may be used include, e.g., FLAG®, HA, HIS,c-Myc, VSV-G, V5, and HSV. Select epitope tags that may be used with thepresent invention are listed below.

TABLE 2 Select Epitope Tag Sequences Tag Sequence SEQ ID NO: HIS HHHHHHSEQ ID NO: 11 c-MYC EQKLISEEDL SEQ ID NO: 19 HA YPYDVPDYA SEQ ID NO: 20VSV-G YTDIEMNRLGK SEQ ID NO: 21 HSV QPELAPEDPED SEQ ID NO: 22 V5GKPIPNPLLGLDST SEQ ID NO: 23 FLAG DYKDDDDK SEQ ID NO: 9

Cell Surface Display Sequence

The construct may comprise a sequence for expression on the cellsurface. For example, after Golgi-derived vesicle to plasma membranefusion occurs where the vesicle contains a substrate (containing a ERtargeting sequence and an ER retention sequence), a cell-surface displaysequence may be used to retain an amino acid (e.g., comprising one ormore phosphorylated or unphosphorylated substrate sequences) on thesurface of a eukaryotic cell, such as, e.g., a yeast cell.

In some embodiments, an Aga2p sequence can be used to display an aminoacid sequence, such as a cleaved or uncleaved substrate amino acidsequence, on the surface of a eukaryotic cell, such as a yeast. Forexample, yeast cells can display a substrate from a randomized libraryextracellularly as a fusion to the Aga2p cell surface mating factor,which is covalently bound to the Aga1p mating factor via disulfide bonds(e.g., see FIG. 2 ). Expression of a fusion construct comprising Aga2pon the surface of yeast. Aga2p is an adhesin protein that is involved inagglutinin interaction mediated by Aga1p-Aga2p complexes and Sag1p(Huang et al., 2009), and Aga2p may be used for extracellular expressionof a fusion protein in yeast (e.g., Kim et al., 2010; Boder and Wittrup,1997). The Aga2p approach for expression of fusion proteins on thesurface of yeast may be used for expression of a wide variety ofproteins (Gai et al., 2007).

In other embodiments, an amino acid sequence, such as a phosphorylatedor unphosphorylated substrate, may be displayed on the cell surface of acell, such as a yeast using a glycosylphosphatidylinositol (GPI) anchorattachment signal sequence.

A mammalian mannose type Man5GlcNAc2 N-linked glycans may also be usedto display a substrate. For example, a glycoengineered Pichia pastorishost strain that is genetically modified to secrete glycoproteins may beparticularly useful for displaying a glycoprotein via this method asdescribed, e.g., in Lin et al. (2011). This surface display method mayuse a linker (e.g., a pair of coiled-coil peptides) while using aGPI-anchored cell surface protein as an anchoring domain, such as, e.g.,the Saccharomyces cerevisiae Sed1p GPI-anchored cell surface protein.

A self-assembled amyloid-like oligomeric-cohesin scaffoldin may be usedfor protein display on a yeast, such as, e.g., Saccharomyces cerevisiae.For example, the cellulosomal scaffolding protein cohesin and itsupstream hydrophilic domain (HD) may be genetically fused with the yeastUre2p N-terminal fibrillogenic domain consisting of residues 1 to 80(Ure2p1-80). The resulting Ure2p1-80-HD-cohesin fusion protein may beexpressed in Escherichia coli to produce self-assembled supramolecularnanofibrils that can serve as a protein scaffold. The excess cohesinunits on the nanofibrils provide ample sites for binding to dockerinfusion protein, such as a dockerin-substrate fusion protein.Self-assembled supramolecular cohesin nanofibrils created by fusion withthe yeast Ure2p fibrillogenic domain can provide a protein scaffold thatcan be effectively used for yeast cell surface display. Related methodsare described in additional detail in Han et al. (2012).

In some embodiments, the construct may comprise an Aga2p sequence. TheAga2p yeast display system (Boder and Wittrup, 1997) has been previouslycharacterized and may be used in various aspects of the presentinvention. Non-limiting examples of proteins that may be used ascell-surface proteins are described in Chen et al. (2011); Lee et al.(2011); Lin et al. (2012); Han et al. (2012); Gai et al. (2007); andarticle in press as: Gera et al. (2012), each of which are incorporatedby reference in their entirety.

Vectors

The term “vector” is used to refer to a carrier nucleic acid moleculeinto which a nucleic acid sequence can be inserted for introduction intoa cell where it can be replicated. A nucleic acid sequence can be“exogenous,” which means that it is foreign to the cell into which thevector is being introduced or that the sequence is homologous to asequence in the cell but in a position within the host cell nucleic acidin which the sequence is ordinarily not found. Vectors include plasmids,cosmids, viruses (bacteriophage, animal viruses, and plant viruses), andartificial chromosomes (e.g., YACs). One of skill in the art would bewell equipped to construct a vector through standard recombinanttechniques. In certain preferred embodiments, the vector can express anucleic acid sequence in a eukaryotic cell, such as, e.g., a yeast cell.

The term “expression vector” refers to any type of genetic constructcomprising a nucleic acid coding for a RNA capable of being transcribed.In some cases, RNA molecules are then translated into a protein,polypeptide, or peptide. In other cases, these sequences are nottranslated, for example, in the production of antisense molecules orribozymes. Expression vectors can contain a variety of “controlsequences,” which refer to nucleic acid sequences necessary for thetranscription and possibly translation of an operably linked codingsequence in a particular host cell. In addition to control sequencesthat govern transcription and translation, vectors and expressionvectors may contain nucleic acid sequences that serve other functions aswell, such as those described herein.

An expression vector may comprise, for example, one or two or morepromoters, enhancers, initiation signals, internal ribosome bindingsites, multiple cloning site (MCS), RNA splicing sites, terminationsignals, polyadenylation signals, origins of replication (often termed“ori”), or selectable or screenable markers.

IV. Expression in Eukaryotic Cells

In certain aspects of the present invention, a kinase and kinasesubstrate may be expressed in eukaryotic cells in the presence of akinase inhibitor. Cells that may be used with the present inventioninclude, e.g., yeast, mammalian cells, insect cells, stem cells, humancells, primate cells, induced pluripotent stem cells, cancerous cells,and embryonic cell lines (e.g., HEK 293 cells, etc.). In someembodiments, yeast cells are used. In some embodiments, the yeast cellsare Kex2 (−/−) knockout yeast cells. In some embodiments, the yeast hasone, two, or all of the SNQ2, YOR1, and/or PDQ5 genes knocked out. SNQ2,YOR1, and PDQ5 can function as small molecule transporters, and byknocking out one or more of these genes (e.g., in a yeast) it may bepossible to reduce the export of a kinase inhibitor from the cell. It isanticipated that, in various embodiments, virtually any cell thatcontains an endoplasmic reticulum (ER) may be used to selectively targeta kinase and a substrate to the ER of the cell.

Using eukaryotic cells, such as yeast, for expression of a protein orenzyme of interest can offer significant advantages over using bacteria.For example, in view of previous experience with E. coli-based proteaseengineering systems (Varadarajan et al., 2008) as well as yeast surfaceexpression (Boder and Wittrup, 1997), the YESS approach uses eukaryoticcells and thus can offer several potential advantages for identifyingmutant kinases that are resistant to a kinase inhibitor. For example,the eukaryotic expression machinery in yeast can be more compatible withmammalian kinases, especially human kinases, as compared with bacteria,such as E. coli.

In some embodiments, yeast cells are used for selection of a mutantkinase that is resistant to a kinase inhibitor. Yeast cells may in someembodiments be advantageously used since, e.g., they are capable ofdividing quickly and are relatively robust and allow for a reasonablysimple culture. Yeast cell lines that may be used with the presentinvention include, e.g., GS115 cells, INVSc1 cells, KM71H cells, SMD1168cells, SMD1168H cells, and X-33 cells. It is anticipated that virtuallyany strain of yeast may be used with the present invention. In someembodiments the yeast may be, e.g., Saccharomyces cerevisiae or Pichiapastoris. The yeast may be an Ascomycota, such as a Saccharomycotina(referred to as “true yeasts”), or a Taphrinomycotina, such asSchizosaccharomycetales (the “fission yeasts”).

Various insect cell lines may be used with the present invention. Forexample, insect cells that may be used with the present inventioninclude, e.g., Drosophila cells, Sf9 cells, and Sf21 cells.

Mammalian cell lines that may be used with the present inventioninclude, e.g., HEK 293 cells, CHO cells, 3T3 cells, BHK cells, CV1cells, Jurkat cells, and HeLa cells. In some embodiments, a human cellline may be used.

V. Cell Sorting

Cells may be sorted based on the presence of one or more sequences onthe surface of the cell. For example, cells may be sorted usingfluorescence-activated cell sorting (FACS) or magnetic-activated cellsorting (MACS).

Subsequent to cell sorting, the specific kinase (e.g., one or moremutant kinases that are resistant to a kinase inhibitor) produced by ayeast may be determined by genotyping nucleic acids from a colony of theyeast. A variety of known methods may be used for nucleotide sequencing.Virtually any sequencing method, such as, for example, traditionalmethods of sequencing or next-generation sequencing methods, may be usedto determine the sequence of a kinase expressed in a cell. In someembodiments, the nucleotide sequencing can be determined, e.g., bypyrosequencing or by chain termination sequencing. In some preferredembodiments, the mutant kinase is sequenced using a next-generationsequencing methodology. For example, in some embodiments, Nanopore orPacBio® sequencing may be used in order to sequence multiple mutationsover a larger portion of the mutant kinase.

Magnetic-Activated Cell Sorting (MACS)

Cells that selectively express a phosphorylated sequence on the surfaceof the cells (e.g., phosphorylated by a kinase expressed by the cell)may be isolated or separated from other cells using a magnetic-activatedcell sorter (MACS). MACS typically utilizes an antibody (e.g., anantibody that selectively binds an epitope tag sequence located withinan expressed protein or peptide), in combination with magnetic beads toseparate cells over a column. MACS may, in certain embodiments, berelatively gentle on cells and favorably affect cell viability andintegrity of certain mammalian cell lines as compared to FACS.

Various MACS products are commercially available, including MACSMicroBeads™ columns or AutoMACS™ (Miltenyi Biotec, CA, USA), and may beused according to the manufacturer's instructions. PBS/0.5% BSA (withoutEDTA) may used as the buffer for cell isolation. In some experiments, aDead Cell Removal Kit (Miltenyi Biotec) may be used to remove dead cellsprior to isolation of cells that express a cleaved target sequence.Repeated MACS columns may be used if necessary.

Fluorescence-Activated Cell Sorting (FACS)

Fluorescence-activated cell sorting (FACS) may also be used to separatecells that express a phosphorylated substrate sequence. FACS utilizesthe degree of fluorescence exhibited by a cell to separate cells. Incertain embodiments, antibodies comprising different fluorescent labelsmay be used to separate or purify a cell, such as a yeast cell, thatexpresses a phosphorylated substrate on the surface of the cell (e.g.,indicating the presence of a mutant kinase that is able to phosphorylatean amino acid in a substrate sequence in the presence of a kinaseinhibitor).

In some embodiments, FACS screening or other automated flow cytometrictechniques may be used for the efficient isolation of a eukaryotic cell(e.g., a yeast cell) comprising a mutant kinase. Instruments forcarrying out flow cytometry are known to those of skill in the art andare commercially available to the public. Examples of such instrumentsinclude FACStar™ Plus, FACScan™, and FACSort™ instruments from BectonDickinson (Foster City, Calif.), Epics C from Coulter Epics Division(Hialeah, FA), and MOFLO™ from Cytomation (Colorado Springs, Colo.).

FACS may be used for sorting of cells. In various embodiments, thepresence or absence of 1, 2, or more antibodies, which recognize 1, 2,or more epitope tags, amino acid sequences, or phosphorylated aminoacids on the surface of a cell, reflects the activity of a kinase. FACSmay also be used to separate cells that have been transformed with adesired construct from cells that do not contain or have not beentransformed with a desired construct.

Flow cytometric techniques in general involve the separation of cells orother particles in a liquid sample. Typically, the purpose of flowcytometry is to analyze the separated particles for one or morecharacteristics, such as, e.g., presence of a labeled ligand or othermolecule. FACS generally involves the direction of a fluid samplethrough an apparatus such that a liquid stream passes through a sensingregion. The particles should pass one at a time by the sensor and arecategorized base on size, refraction, light scattering, opacity,roughness, shape, fluorescence, etc.

Rapid quantitative analysis of cells proves useful in biomedicalresearch and medicine. Apparatuses permit quantitative multiparameteranalysis of cellular properties at rates of several thousand cells persecond. These instruments provide the ability to differentiate amongcell types. Data are often displayed in one-dimensional (histogram) ortwo-dimensional (contour plot, scatter plot) frequency distributions ofmeasured variables. The partitioning of multiparameter data filesinvolves consecutive use of interactive one- or two-dimensional graphicsprograms.

Quantitative analysis of multiparameter flow cytometric data for rapidcell detection consists of two stages: cell class characterization andsample processing. In general, the process of cell classcharacterization partitions the cell feature into cells of interest andnot of interest. Then, in sample processing, each cell is classified inone of the two categories according to the region in which it falls.

FACS is described further, e.g., in U.S. Pat. Nos. 3,826,364; 4,284,412;4,989,977; 4,498,766; 5,478,722; 4,857,451; 4,774,189; 4,767,206;4,714,682; 5,160,974; and 4,661,913, each of which are specificallyincorporated herein by reference.

In some embodiments, flow cytometry can be used repeatedly duringmultiple rounds of screening that are carried out sequentially. Cellsmay be isolated from an initial round of sorting and immediatelyreintroduced into the flow cytometer and screened again to improve thestringency of the screen. In some embodiments, non-viable cells can beadvantageously recovered or separated using flow cytometry. Since flowcytometry generally involves a particle sorting technology, the abilityof a cell to grow or propagate is not necessary in various embodimentsof the present invention. Techniques for the recovery of nucleic acidsfrom such non-viable cells are well known in the art and may include,for example, use of template-dependent amplification techniques,including PCR.

Bioreactors and Robotic Automation

One or more steps for the culture or separation of cells may beautomated. Automating a process using robotic or other automation canallow for more efficient and economical methods for the production,culture, and differentiation of cells. For example, robotic automationmay be utilized in conjunction with one or more of the culture ofeukaryotic cells, passaging, addition of media, and separation of cellsexpressing a cleaved or uncleaved substrate, e.g., using MACS or FACS.

A bioreactor may also be used in conjunction with the present inventionto culture or maintain cells. Bioreactors provide the advantage ofallowing for the “scaling up” of a process in order to produce anincreased amount of cells. Various bioreactors may be used with thepresent invention, including batch bioreactors, fed batch bioreactors,continuous bioreactors (e.g., a continuous stirred-tank reactor model),and/or a chemostat. A bioreactor may be used, e.g., to produce increasednumbers of eukaryotic cells, such as yeast.

VI. Next Generation Sequencing

A variety of next generation-sequencing systems may be used with thepresent invention include. For example, the next-generation sequencermay utilize single-molecule real-time sequencing (e.g., produced byPacific Biosciences, Menlo Park, Calif.), an ion semiconductor method(e.g., Ion Proton™, Ion PGM™), a pyrosequencing method (e.g., 454), asequencing by synthesis method (e.g., an Illumina™ sequencer), or asequencing by ligation method (e.g., a SOLiD™ sequencer). In someembodiments, the next generation sequencer is an Illumina™ sequencingsystem, or an Ion Torrent system (e.g., the Ion Proton™ Sequencer or theIon PGM™ sequencer) from Life Technologies (Carlsbad, Calif., USA),SOLID, SOLID 2.0, 5500 Genetic Analyzer (e.g., 5500, 5500 W, etc.; LifeTechnologies, Carlsbad, Calif.) may be used in various embodiments ofthe present invention. In some embodiments, the next generationsequencer is a Pacific Biosciences system (e.g., the Sequel System orthe PacBio RSII). In some embodiments, an automated method for samplepreparation may be used; for example, the Ion Chef™ system may be used,e.g., in combination with an ion semiconductor sequencer such as, e.g.,Ion Proton™ or Ion PGM™ (e.g., using the Ion 314™ Chip, Ion 316™ Chip,Ion 318™ Chip Ion PI™ Chip, or Ion PII™ Chip). Various Illumina systemsare available and may be used in embodiments of the present inventionsuch as, e.g., the HiSeq X Ten, HiSeq 2500, NextSeq 500, and MiSeqsystems. The next-generation sequencing method may involve constructinga library by generating DNA, fragmenting the DNA, and then addingadaptors. Then the fragmented DNA may be amplified on beads, e.g., usingemulsion PCR. In some embodiments, the next-generation sequencing methoddoes not utilize beads (e.g., 5500 W, Illumina sequencers, etc.). It isanticipated that in some embodiments, amplification of sequences may beaccomplished on a glass surface or solid support.

A. Data Analysis

Data obtained regarding the sequences of a resistant kinase, or a kinasethat can continue to function in the presence of a kinase inhibitor, maybe aligned with the sequence of the wild-type kinase in order toidentify the mutations in the resistant kinase using a variety ofalignment techniques and programs that are well-known in the art. Insome embodiments, sequences comprising a stop codon may be excluded.Statistical analysis can be performed to identify the frequency ofmutations which are associated with resistance of a kinase to a kinaseinhibitor. In some embodiments, resistance mutations for a kinase may becompared to previously identified mutations that can also provideresistance against a kinase inhibitor. As shown in the below examples,the methods described herein can be used to identify resistancemutations that have been observed clinically in vivo.

In some embodiments, data analysis comprises trimming low-quality basecalls from reads (e.g., based on Qphred quality scores below 25), andsequences are aligned to the wild-type sequence in order to identifymutations. Then, mutations may be translated into amino acid changes.The amino acid changes may be counted both individually and as theyoccur as compound mutations. The aligned and translated data of theunsorted pool may then be used to assess library quality and/or the datamay be used as a baseline to calculate enrichment of mutations in thesorted libraries. In the case of comparing occurrences of compoundmutations to single mutations, enrichment factors may in some instancesbe more informative because they can account for initial frequency of aparticular mutation.

VII. EXAMPLES

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventor to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

Example 1 Materials and Methods Vector Construction

Amino acids 237-630 of human ABL1 isoform 1 were cloned into the pESDvector (Yi et al. 2014) under the GAL10/GAL1 bidirectional promoter inplace of TEV protease. For the library template vector, the TEV proteasesubstrate was replaced with a minimal ABL substrate (AAAAAYAAAAA; SEQ IDNO:1). Yeast receptor adhesion subunit Aga2, ER retention signal, andhexahistidine (6-His) and FLAG epitope tags were retained from the pESDvector.

Validation of YESS-Based Inhibitor Resistance Assay

ABL wild-type and T315I mutant cultures were induced by growth in SG-UTmedium containing 125 μM Dasatinib, Ponatinib, or equivalent volume ofDMSO. After 40 hours of growth at 20 degC, cells were washed three timeswith TBS+0.5% BSA+0.05% Tween20. Cells were stained with anti-His6-FITC(ThermoFisher) and anti-phosphotyrosine-PE (BioLegend) at 4° C. for 30minutes, followed by three washes with TBS+0.5% BSA+0.05% tween20. FACSanalysis was performed on either the FACS Aria Ilu or FACSCalibur™ (BDBiosciences).

Error-Prone Library Construction

ABL kinase was amplified with primers JT017(5′-ACCTCTATACTTTAACGTCAAGGAG-3′; SEQ ID NO:2) and JT119(5-′GGTAACGGAACGAAAAATAGAAAAGGATATTACATGGG-3′; SEQ ID NO:24) to producea 1550 bp product. A12 polymerase, an error-prone variant of KODpolymerase, was used for amplification to generate a pool of mutants(FIG. 1 ). Vector was prepared by digestion with SalI-HF, XhoI, andNcoI-HF. PCR product and digested vector were column purified and dropdialyzed in ddH₂O on VSWP membranes for one hour. ElectrocompetentEBY100 were prepared as described previously (Yi et al. 2014; Boder andWittrup 1997; Benatuil et al., 2010; Perez et al., 2010). In each ofthree 2 mm electroporation cuvette, 350 μl electrocompetent EBY100 werecombined with 10 μg PCR product from error prone PCR and 3 μg digestedpESD-derived vector to a maximum volume of 400 μl (FIG. 1 ). Transformedcells were passaged in SD-UT medium three times before proceeding tosorting experiments. Library size was estimated by colony counts fromdilution series of transformed cells plated on SD-UT agar. Sangersequencing was carried out on 32 randomly selected clones, as well ashigh throughput sequencing (described below) on an aliquot of the entiresample.

Library Screening by FACS

Library cells were induced by growth in 10 mL SG-UT with 25 μM inhibitoror equivalent volume of DMSO. 5×10⁷ cells were washed with TBS+BSA thenstained with anti-His6-FITC and anti-pY-PE for 30 minutes at 4° C. Cellswere washed three times to remove unbound antibody. Wild-type ABL withand without inhibitor was used to determine the location of the sortinggates. PE+/FITC+ cells were sorted and re-sorted, then transferred toSD-UT medium for growth at 30° C. until dense, 1-2 days depending onnumber of cells collected (FIG. 1B). Subsequent rounds were performedidentically until PE+/FITC+ cells accounted for 60-90% of thepopulation.

High-Throughput Sequencing

Plasmids were recovered from saturated overnight cultures using ZymoPrepII kit (Zymo Research). DNA from unsorted, Dasatinib-, andPonatinib-sorted libraries was barcoded on both ends using primers withidentical annealing sequences but unique 16-mer sequences. Afterbarcoding PCR, concentrations were quantified by Qubit and samples werepooled in equimolar ratios. Samples were sequenced using the PacBio RSIIat the Arizona Genomics Institute at the University of Arizona in orderto generate long reads so as to identify any compound mutations in thesame clone (FIG. 1B).

Sequence Analysis

Sequences were assigned to their origin based on barcodes. Those withouta 5′ or a 3′ barcode were discarded. Sequences were aligned to wild-typeABL kinase domain using an implementation of NCBI BLAST on the TexasAdvanced Computing Core. Mutations in aligned sequences were thentranslated and compiled into a database.

YESS Validation of Resistant Mutants

Mutations to be validated were selected from the most frequentlyrecovered sequences in each pool. Mutations were then introduced usingmismatched primers followed by overlap-extension PCR. Sequence-validatedclones were then grown overnight in SD-UT medium followed by inductionin SG-UT medium with 25 μM inhibitor or DMSO for two days at 20° C.Cells were stained as previously described and analyzed on aFACSCalibur™ HTS (FIG. 1C).

In Vitro Validation of Resistant Mutants

ABL kinase domain (AA 229-511) was expressed as previously described(Kuriyan, Protein Science). Cleared cell lysate was applied to 2 mLequilibrated Ni-NTA resin in a gravity-flow column. Elution fractionswere dialyzed overnight into QA buffer (Kuriyan, Protein Science),followed by anion-exchange purification with a Mono Q 5/50 GL column (GELife Science). Kinetic activity was assayed by incorporation of[γ-³²P]-ATP (Sigma Aldrich) into the known substrate ABLtide (AnaSpec).Reactions contained 200 μM ABLtide, 100 mM Tris pH8.0, 10 mM MgCl₂, and2 mM DTT. K_(m)(ATP) was first determined using saturating substrate(200 μM), followed by K_(i) determination with [ATP] at K_(m)(ATP).

Ba/F3 Validation of Resistance Mutants

BCR/ABL-transduced Ba/F3 cells were grown in RPMI medium (Lonza)supplemented with 10% FBS, 100 U/mL penicillin, and 100 ug/mLstreptomycin. Cells thawed from freezer stocks were passaged twice inmedia additionally supplemented with 10 ng/mL IL-3 (Peprotech), followedby two passages in the absence of IL-3. 5×10⁴ cells were seeded in 100μl in each well of a 96-well plate. 50 μl of inhibitor diluted inRPMI+FBS+Pen/Strep was added to each well. After 24 hours, cellviability was quantified using the CellTiter Glo kit (Promega) accordingto manufacturer's instructions, with the exception that the reagent wasdiluted 1:5 in sterile PBS. Luminescence was detected using 96-wellplate reader (Tecan). Titraiton curves were fitted using afour-parameter dose-response curve using GraphPad Prism (FIG. 5 ).

Example 2 Identification of Resistant Mutants Validation of YESS-BasedInhibitor Resistance Assay

FACS analysis of wild-type ABL kinase expressing cells showed a decreasein PE fluorescence in the presence of 125 μM Dasatinib or Ponatinibcompared to DMSO only. ABL T315I expressing cells showed a decrease inPE signal in the presence of Ponatinib but not Dasatinib compared toDMSO only (FIG. 2 ). These data correspond to known resistance of ABLT315I to inhibition by Dasatinib but not Ponatinib.

Error-Prone Library Construction

An error-prone library was created as described above. Library size wasestimated to be 3.2×10⁷ based on colony counts. This library sizeexceeds single point mutation diversity by approximately 10⁴-fold anddouble mutation diversity by two-fold. Sanger sequencing of 32individual clones showed a mutation rate of 0.07%, or about one mutationper gene, on average. High-throughput sequencing generated approximately2×10⁴ sequence reads containing 1.4×10⁴ mutations. 77% of all possiblesingle nucleotide substitutions were observed.

Library Screening by FACS

The fraction of PE-positive cells in Dasatinib-treated culturesincreased from 1.8% in the unsorted pool to 87.7% in the post-sort 6sample. The fraction PE-positive cells in Ponatinib treated culturesincreased from 0.31% in the unsorted pool to 57.9% of cells in thepre-sort 4 pool. 10⁷ Dasatinib-treated cells were sorted and 5×10⁴ cellswere collected in the first round. 3.3×10⁷ Ponatinib-treated cells weresorted and 2.6×10⁴ of these were collected in the first round. Insubsequent rounds, the number of cells sorted always exceeded the firstbottleneck by at least 10-fold.

Sequence Analysis

PacBio RSII returned approximately 10⁴ sequence reads for each of thethree pools: unsorted library, Ponatinib-screened, andDasatinib-screened. Due to the high-frequency of insertions anddeletions in the PacBio data, sequences were aligned to the wild-typeABL kinase domain. In each case the sorted pools were highly polarizedcompared to the unsorted pool. In the Dasatinib-screened pool, the top73 mutants composed over 80% of the observed sequences (Table 3, partA). In the Ponatinib-screened pool, the top 10 mutants composed over 80%of the observed sequences (Table 3, part B). In the unsorted library,the top 1250 mutants account for 80% of the observed sequences.

TABLE 3 Dasatinib and Ponatinib Screen Sequencing Results Rank MutationsCount IC50(nM) A Dasatinib Screen Parental 5623 0.8-5.6 1 V448L 1241 2E255V 773 6.3-11  3 F317L 625 7.4-18  4 G250E 532 1.8-8.1 5 Y253H 5241.3-10  6 F493L 422 7 D455G 347 8 V448M 306 9 R239H 198 10 F401L 157 11F497L 143 12 L384M 122 4 13 Q252H 108 3.4-5.6 14 F359I 82 15 V304A 82 16E255V/V448L 76 17 V299L 71 15.8-44.1 18 F317L/V448L 66 19 Y253H/V448L 5920 F317L/F493L 57 . . . . . . . . . 49 T315I 21  137->1000 52 M244V 201.3-3.6 383 F359V 3 2.2-2.7 423 T392I 2 4.57 716 G303R 1 41.92 n/a F311I0 2.7 n/a M351T 0 1.1-1.6 n/a H396R 0 1.3-3.0 n/a D325N 0 5.84 n/aE255V/G303R 0 38 n/a E255V/D325N 0 32.78 n/a E255V/T392I 0 7.3 BPonatinib Screen Parental 1662  3.9-15.8 1 E255V 2605 41.9-55.6 2 Y253H751 29.8 3 E255V/M351L/G555D 419 4 E255V/M351L 373 5 E255V/G555D 279 6E255V/E450G 199 7 Y253H/E255V 182 203.5 8 G303R 172 89.8 9 E255V/T392I109 40.5 10 G555D 83 11 E255V/V448M 79 12 M351L 73 13 E255V/G303R 7056.1 14 K285N 67 15 M351L/G555D 55 16 E255V/K415E 47 17 E255V/T597I 4618 Y253H/G555D 45 19 G250E 38 39.3 20 E450G 38 . . . . . . . . . 24E255V/D325N 31 294.2 35 T392I 18 17 73 D325N 8 21.2 103 T315I 5 29.1 230Q252H 2 27 233 M244V 2 12.7 n/a V299L 0 8.5 n/a F311I 0 13.4 n/a F317L 013.8 n/a M351T 0 9 n/a F359V 0 22.7 n/a H396R 0 20.1

The mutations recovered from this screen were compared to mutations seenin patients treated for CML. From the Dasatinib-screened pool, four ofthe five most frequent mutations observed are known mutations that havebeen recovered from CML patients (FIG. 4 , top left). In the case ofPonatinib, all five of the common mutations from this data are known tobe present in CML patients. In addition, six of the top 10 most commonsequence reads from the Ponatinib-screened pool contained multiplemutations (FIG. 4 , top right). 23% of all sequence reads contained twomutations, representing a 4-fold increase from the unsorted pool (FIG. 4, bottom). The presence of multiple mutations reflects data seen fromCML patients with multiple mutations in the ABL gene. Double mutantshave been show in vitro to be resistant to inhibition by Ponatinib,which is otherwise highly effective against single kinase inhibitorresistant mutations (Zabriske et al. 2014, Cell).

In Vitro Characterization of Mutants

Seven kinase-domain mutants selected for Ponatinib resistance andwild-type BCR-ABL were assayed for resistance to inhibition by Ponatinibin the murine Ba/F3 pro-B cell line. In particular, three commonPonatinib-resistant double mutation variants and the corresponding fourconstitutive single mutation analogs were assayed to investigate theimportance of the compound mutations for Ponatinib resistance (FIG. 5 ).All three of the double mutants (E255V/G303R, E255V/D325N, E255V/T392I)assayed had a significantly higher IC₅₀ values for Ponatinib thanwild-type. The two single mutants which were in the top 20 sequences(E255V, G303R) also displayed higher IC₅₀ values compared to wild-type,while the two single mutants which were not in the top 20 sequences didnot have significantly higher IC₅₀ values for Ponatinib (D325N, T392I).The E255V/G303R double mutant was slightly less resistant than thesingle G303R mutation, but more resistant than the E255V mutation alone.The E255V mutation conferred a 2.5-fold increase in IC₅₀ for Ponatinib,while the D325N mutation alone was not significantly different fromwild-type. However, when combined, these two mutations result in an IC₅₀nearly 20-fold higher than wild-type. On the other hand, the E255V/T392Idouble mutant does not confer additional resistance compared to its mostresistant constitutive mutation, E255V, while the T392I mutation aloneis no more resistant than wild-type.

Example 3 Identification of Kinase Inhibitor Resistant BTK MutantsVector Construction

Expressing BTK with the initial vector was toxic to Saccharomycescerevisiae. This toxicity was overcome by expressing BTK using a weakersynthetic promoter taken from the p416leum-A2-G4BS12 plasmid (Blazeck etal. 2012). To accommodate this new promoter the kinase cassette and thedisplayed phospho-acceptor reporter cassette were split across twodifferent plasmids. The kinase cassette including the promoter fromp4161eum-A2-G4BS12 plasmid and the nucleotide sequence encoding humanBTK (217-655; i.e., Genbank reference number 695, amino acids 217-655,corresponding to nucleotides 649-1965) were subcloned into a plasmidcontaining the yeast CEN6 centromere, a yeast G418 resistance marker, anE. coli ColE1 origin of replication and an E. coli ampicillin resistancemaker. This vector was referred to as: pCMYpLeumG4B S12_SH3_SH2_BTK(FIG. 6 ).

The phospho-acceptor reporter encodes the 18 amino acids(ERDINSLYDVSRMYVDPS, SEQ ID NO: 25) of 1-phosphatidylinositol4,5-bisphosphate phosphodiesterase gamma-2 (PLCG2) which arephosphorylated by BTK. This vector was referred to as:pESD_PLCγ2_reporter (FIG. 7 ). Yeast receptor adhesion subunit Aga2, ERretention signal, and hexahistidine (6-His) and FLAG epitope tags wereretained from the pESD vector.

Validation of YESS-Based Inhibitor Resistance Assay for BTK.

Human BTK (217-655) wild-type and C481S mutant cultures were induced bygrowth in SG-UT medium plus 100 μg/mL G418 containing 400 nM Ibrutinib,1000 μM Acalabrutinib, or equivalent volume of DMSO. After 48 hours ofgrowth at 25° C., cells were washed three times with TBS+0.5% BSA. Cellswere stained with anti-His6-FITC (ThermoFisher) and Alexa Fluor® 647anti-Phosphotyrosine Antibody (BioLegend) at 4 degC for 30 minutes,followed by three washes with TBS+0.5% BSA. FACS analysis was performedon either the FACS Aria II (BD Biosciences). Results of subsequentrounds (Native, R2, R3) of YESS screening is shown in FIG. 8 .

Error-Prone Library Construction

BTK (217-655) was amplified with primers JD1181(5′-ATTAACGGAAGCTTcggattagaagccg-3′; SEQ ID NO: 26); and JD1185(5′-gtgttactactcgttattattgcgtattttgtgatgc-3′; SEQ ID NO: 27) to producea 1735 bp product. A12 polymerase, an error-prone variant of KODpolymerase, was used for amplification to generate a pool of mutants.Vector was prepared by digestion with XhoI and XbaI. PCR product anddigested vector were gel extracted and drop dialyzed in ddH₂O on VSWPmembranes for one hour. Electrocompetent EBY100 harboring thepESD_PLCγ2_reporter plasmid were prepared as described previously withthe exception that YPD media was replaced with SD-UT to maintain thepESD_PLCγ2_reporter plasmid (Yi et al. 2014; Boder and Wittrup 1997;,Benatuil et al., 2010; Perez et al., 2010)). In each of six 2 mmelectroporation cuvette, 350 μl electrocompetent EBY100 cells harboringpESD_PLCγ2_reporter were combined with 10 μg PCR product from errorprone BTK PCR and 3 μg digested pCMYpLeumG4BS12_SH3_SH2_BTK vector to amaximum volume of 400 μl. Transformed cells recovered in SD-UT for 4hours at 30° C. before they were passaged in SD-UT medium plus 100 μg/mLG418 two times before proceeding to sorting experiments. Library sizewas estimated by colony counts from dilution series of transformed cellsplated on SD-UT agar plus 100 μg/mL G418. Sanger sequencing was carriedout on 15 randomly selected clones, as well as high throughputsequencing (described below) on an aliquot of the entire sample.

Library Screening by FACS

Library cells were induced by growth at 25° C. for 45 hr in 50 mL SG-UTplus 100 μg/mL G418 with 400 nM Ibrutinib, 1000 μM Acalabrutinib orequivalent volume of DMSO. 3×10⁸ cells were washed with TBS+BSA for eachlibrary and then stained with anti-His6-FITC and anti-pY-APC for 30minutes at 4° C. Cells were washed three times to remove unboundantibody. Wild-type BTK (217-655) with and without inhibitor was used todetermine the location of the sorting gates. APC+/FITC+ cells weresorted, then transferred to SD-UT medium plus 100 μg/mL G418 for growthat 30° C. until dense, 1-2 days depending on number of cells collected(FIG. 7 ).

High-Throughput Sequencing

Plasmids were recovered from saturated overnight cultures using ZymoPrepII kit (Zymo Research). DNA from unsorted, Ibrutinib-, andAcalabrutinib-sorted libraries were barcoded on both ends using primerswith identical annealing sequences but unique 16-mer sequences. Afterbarcoding PCR, concentrations were quantified by Qubit and samples werepooled in equimolar ratios. Samples were sequenced using the PacBio RSIIat the University of California, Davis DNA Technologies Core in order togenerate long reads.

Validation of YESS-based Inhibitor Resistance Assay

FACS analysis of wild-type human BTK kinase (217-655) was performed.Kinase expressing cells showed a decrease in APC fluorescence in thepresence of 400 nM Ibrutinib or 1000 μM Acalabrutinib compared to DMSOonly. BTK C481S expressing cells retained APC signal in the presence ofIbrutinib and Acalabrutinib compared to DMSO only (FIG. 9 ). These datacorrespond to known resistance of BTK C481S to inhibition by Ibrutiniband Acalabrutinib.

Error-Prone Library Construction

An error-prone library was created as described above. Library size wasestimated to be 1.3×10⁷ based on colony counts. This library sizeexceeds single point mutation diversity by approximately 10⁴-fold.Sanger sequencing of 16 individual clones showed a mutation rate of0.08%, or about one mutation per gene, on average.

Library Screening by FACS

The fraction of APC-positive cells in Ibrutinib-treated culturesincreased from 0.7% in the unsorted pool to 40.60% pre-sort round 3. Thefraction of APC-positive cells in Acalabrutinib treated culturesincreased from 1.63% in the unsorted pool to 23.8% of cells in thepre-sort round 3 pool. These collected cells were restained and resortedthe same day to generate a pool of cells with kinase activity. 1.1×10⁸Ibrutinib-treated cells were sorted in the first round. 0.9×10⁸Acalabrutinib-treated cells were in the first round.

All of the methods disclosed and claimed herein can be made and executedwithout undue experimentation in light of the present disclosure. Whilethe compositions and methods of this invention have been described interms of preferred embodiments, it will be apparent to those of skill inthe art that variations may be applied to the methods and in the stepsor in the sequence of steps of the method described herein withoutdeparting from the concept, spirit and scope of the invention. Morespecifically, it will be apparent that certain agents which are bothchemically and physiologically related may be substituted for the agentsdescribed herein while the same or similar results would be achieved.All such similar substitutes and modifications apparent to those skilledin the art are deemed to be within the spirit, scope and concept of theinvention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

-   U.S. Pat. No. 3,826,364-   U.S. Pat. No. 4,284,412-   U.S. Pat. No. 4,498,766-   U.S. Pat. No. 4,554,101-   U.S. Pat. No. 4,661,913-   U.S. Pat. No. 4,714,682-   U.S. Pat. No. 4,767,206-   U.S. Pat. No. 4,774,189-   U.S. Pat. No. 4,857,451-   U.S. Pat. No. 4,897,268-   U.S. Pat. No. 4,989,977-   U.S. Pat. No. 5,075,109-   U.S. Pat. No. 5,160,974-   U.S. Pat. No. 5,478,722-   U.S. Pat. No. 5,552,157-   U.S. Pat. No. 5,565,213-   U.S. Pat. No. 5,567,434-   U.S. Pat. No. 5,738,868-   U.S. Pat. No. 5,795,587-   U.S. Pat. No. 8,945,855-   U.S. Pat. No. 9,546,359-   PCT/U.S. Ser. No. 15/55494-   WO 2008/137475-   WO 2014/004540 (PCT/US2013/047663)-   Aharoni et al., Chem. Biol., 12(12):1281-1289, 2005.-   Arber et al., J. Cell Biol., 116:113-125, 1992.-   Aridor, M., and Hannan, L. (2000) Traffic jam: a compendium of human    diseases that affect intracellular transport processes, Traffic    (Copenhagen, Denmark) 1, 836-851.-   Aridor, M., and Hannan, L. (2002) Traffic jams II: an update of    diseases of intracellular transport,-   Traffic (Copenhagen, Denmark) 3, 781-790.-   Beinfeld, M. (1998) Prohormone and proneuropeptide processing.    Recent progress and future challenges, Endocrine 8, 1-5.-   Benatuil et al., Protein Eng. Des. Sel., 23(4):155-159, 2010.-   Blazeck, J., et al., Controlling promoter strength and regulation in    Saccharomyces cerevisiae using synthetic hybrid promoters.    Biotechnol Bioeng, 109(11): p. 2884-95, 2012.-   Boder and Wittrup, Nat. Biotechnol., 15(6):553-557, 1997.-   Bostian, K., Elliott, Q., Bussey, H., Burn, V., Smith, A., and    Tipper, D. (1984) Sequence of the preprotoxin dsRNA gene of type I    killer yeast: multiple processing events produce a two-component    toxin, Cell 36, 741-751.-   Boulware, K. T. and Daugherty, P. S. (2006) Protease specificity    determination by using cellular libraries of peptide substrates    (CLiPS), Proc. Nat. Acad. Sci., USA 103, 7583-7588.-   Bourbonnais, Y., Ash, J., Daigle, M., and Thomas, D. (1993)    Isolation and characterization of S. cerevisiae mutants defective in    somatostatin expression: cloning and functional role of a yeast gene    encoding an aspartyl protease in precursor processing at monobasic    cleavage sites, The EMBO journal 12, 285-294.-   Cawley, N., Chen, H., Beinfeld, M., and Loh, Y. (1996) Specificity    and kinetic studies on the cleavage of various prohormone mono- and    paired-basic residue sites by yeast aspartic protease 3, The Journal    of biological chemistry 271, 4168-4176.-   Chanalia et al., Rev. Med. Microbiol., 22(4):6, 2011.-   Chao et al., Nat. Protoc., 1(2):755-768, 2006.-   Chen et al., Proc. Natl. Acad. Sci. USA, 108(28):11399-11404, 2011.-   Collen and Lijnen, Blood, 78(12):3114-3124, 1991.-   Copic et al., Genetics, 182:757-769, 2009.-   Craik et al., Biochem. J., 435(1):1-16, 2011.-   Denecke et al., EMBO J., 11(6):2345-2355,-   Diamond, S. (2007) Methods for mapping protease specificity, Current    opinion in chemical biology 11, 46-51.-   Dix, M. M., Simon, G. M., Cravatt, B. F., (2008) Global Mapping of    the Topography and Magnitude of Proteolytic Events in Biological    Systems, Cell 134, 679-691-   Dougherty and Parks, Virology, 172145, 1989.-   Dougherty et al., Embo J., 7(5):1281-1287, 1988.-   Dougherty et al., Virology, 172:302, 1989.-   Drag and Salvesen, Nat. Rev. Drug Discov., 9:690-701, 2010.-   Drummond et al., J. Mol. Biol., 350(4):806-816, 2005.-   Escalona et al., Nat Rev Genet. 17(8): 459-469, 2016.-   Espada and Martín-Pérez, Int Rev Cell Mol Biol. 331:83-122, 2017.-   Gagnon-Arsenault, I., Tremblay, J., and Bourbonnais, Y. (2006)    Fungal yapsins and cell wall: a unique family of aspartic peptidases    for a distinctive cellular function, FEMS yeast research 6, 966-978.-   Gal et al., Curr. Opin. Struct. Biol., 17:467-473, 2007.-   Gera et al., Methods, Methods. 60(1):15-26, 2013.-   Girard, V., Dieryckx, C., Job, C., and Job, D. (2013) Secretomes:    The fungal strike force, Proteomics 13, 597-608.-   Gould and Tawfik, Biochemistry, 44(14):5444-5452, 2005.-   Gray et al., Cell, 142(4):637-646, 2010.-   Gupta et al., Appl. Microbiol. Biotechnol., 59(1):15-32, 2002.-   Han et al., Appl. Environ. Microbiol., 78(9):3249, 2012.-   Hedstrom, Chem. Rev., 102(12):4501-4524, 2002.-   Hegde and Keenan, Nat Rev Mol Cell Biol., 12(12):787-98, 2011.-   Hegde and Keenan, Nat Rev Mol Cell Biol., 12(12):787-98, 2011.-   Huang et al., Genetics, 182(1):173-89, 2009.-   Jung et al., Proc. Natl. Acad. Sci. U.S.A, 107:604-609, 2010.-   Kapust et al., Biochem. Biophys. Res. Commun., 294:949-955, 2002a.-   Kim et al., Anal. Biochem., 284(1):42-48, 2000.-   Kim et al., Appl Microbiol Biotechnol., 88(4):893-903, 2010.-   Komano, H., Rockwell, N., Wang, G., Krafft, G., and    Fuller, R. (1999) Purification and characterization of the yeast    glycosylphosphatidylinositol-anchored, monobasic-specific aspartyl    protease yapsin 2 (Mkc7p), The Journal of biological chemistry 274,    24431-24437.-   Komano, H., Seeger, M., Gandy, S., Wang, G., Krafft, G., and    Fuller, R. (1998) Involvement of cell surface    glycosyl-phosphatidylinositol-linked aspartyl proteases in    alpha-secretase-type cleavage and ectodomain solubilization of human    Alzheimer beta-amyloid precursor protein in yeast, The Journal of    biological chemistry 273, 31648-31651.-   Kolaczkowska et al., FEBS Lett. Mar. 19; 582(6): 977-983, 2008.-   Kyte and Doolittle, J. Mol. Biol., 157(1):105-132, 1982.-   Ledgerwood, E., Brennan, S., Cawley, N., Loh, Y., and    George, P. (1996) Yeast aspartic protease 3 (Yap3) prefers    substrates with basic residues in the P2, P1 and P2′ positions, FEBS    letters 383, 67-71.-   Lee et al., Bioresource Tech., 102:9179-9184, 2011.-   Levy et al. “Attacking a Moving Target: Understanding Resistance and    Managing Progression in EGFR-Positive Lung Cancer Patients Treated    With Tyrosine Kinase Inhibitors.” Oncology (Williston Park),    30(7):601-12, 2016.-   Li et al. “Profiling Protease Specificity: Combining Yeast ER    Sequestration Screening (YESS) with Next Generation Sequencing.” ACS    Chem Biol. 12(2):510-518, 2017.-   Li, Q., Yi, L., Marek, P., and Iverson, B. (2013) Commercial    proteases: present and future, FEBS letters 587, 1155-1163.-   Lim et al., J. Biol. Chem., 282(13):9722-9732, 2007.-   Lin et al., J. Immunol. Methods, 375:159-165, 2012-   Marnett and Craik, Trends Biotechnol., 23(2):59-64, 2005.-   Matthews, D., Goodman, L., Gorman, C., and Wells, J. (1994) A survey    of furin substrate specificity using substrate phage display,    Protein science: a publication of the Protein Society 3, 1197-1205.-   MEROPS database (merops.sanger.ac.uk)-   Mohanty et al., Protein Expr. Purif., 27:109-114, 2003.-   Monnat et al., Molec. Biol. Cell, 11:3469-3484, 2000.-   Nallamsrtty et al., Protein Expr. Purif., 38(1):108-15, 2004.-   O'Donoghuel, A. J., Eroy-Reveles, A. A., Knudsen, G. M., Ingram, J.,    Zhoul, M., Statnekovl, Alexander, J. B., Greninger, L.,    Hostetterl, D. R., Qu, G., Maltby, D. A., Anderson, M. O.,    DeRisi, J. L., Burlingame, J. A, and Craik, C., (2012) Global    Identification of Peptidase Specificity by Multiplex Substrate    Profiling, Nat Methods 9, 1095-1100.-   O'Loughlin et al., Mol. Biol. Evol., 23(4):764-772, 2006.-   Olsen, V., Cawley, N., Brandt, J., Egel-Mitani, M., and    Loh, Y. (1999) Identification and characterization of Saccharomyces    cerevisiae yapsin 3, a new member of the yapsin family of aspartic    proteases encoded by the YPS3 gene, The Biochemical journal 339 (Pt    2), 407-411.-   Overall and Blobel, Nat. Rev. Mol. Cell Biol., 8(3):245-257, 2007.-   Paltridge, J., Belle, L., and Khew-Goodall, Y. (2013) The secretome    in cancer progression, Biochimica et biophysica acta.-   Park and Rapoport, Annu Rev Biophys., 41:21-40, 2012.-   Pelham et al., Embo J., 7(6):1757-1762, 1988.-   Phan, J., Zdanov, A., Evdokimov, A., Tropea, J., Peters, H., Kapust,    R., Li, M., Wlodawer, A., and Waugh, D. (2002) Structural basis for    the substrate specificity of tobacco etch virus protease, The    Journal of biological chemistry 277, 50564-50572.-   Porro, D., Sauer, M., Branduardi, P., and Mattanovich, D. (2004)    Recombinant protein production in yeasts, METHODS IN MOLECULAR    BIOLOGY- . . . 31, 245-259.-   Ramachandran et al., Nat. Rev. Drug Discov., 11(1):69-86, 2012.-   Rapoport, Nature, 450(7170):663-9, 2007.-   Remington: The Science and Practice of Pharmacy, 21^(st) Ed.    Lippincott Williams and Wilkins, 2005.-   Remington: The Science and Practice of Pharmacy, 21^(st) Ed.,    Pharmaceutical Press, 2011.-   Rockwell, N., and Fuller, R. (1998) Interplay between S1 and S4    subsites in Kex2 protease: Kex2 exhibits dual specificity for the P4    side chain, Biochemistry 37, 3386-3391.-   Rockwell, N., Wang, G., Krafft, G., and Fuller, R. (1997) Internally    consistent libraries of fluorogenic substrates demonstrate that Kex2    protease specificity is generated by multiple mechanisms,    Biochemistry 36, 1912-1917.-   Roebroek, A., Umans, L., Pauli, I., Robertson, E., van Leuven, F.,    Van de Ven, W., and Constam, D. (1998) Failure of ventral closure    and axial rotation in embryos lacking the proprotein convertase    Furin, Development (Cambridge, England) 125, 4863-4876.-   Rozan, L., Krysan, D., Rockwell, N., and Fuller, R. (2004)    Plasticity of extended subsites facilitates divergent substrate    recognition by Kex2 and furin, The Journal of biological chemistry    279, 35656-35663.-   Schechter and Berger, A Biochem. Biophys. Res. Commun.,    27(2):157-162, 1967.-   Schilling and Overall, Nat. Biotechnol., 26(6):685-694, 2008.-   Scholle, M., Kriplani, U., Pabon, A., Sishtla, K., Glucksman, M.,    and Kay, B. (2006) Mapping protease substrates by using a    biotinylated phage substrate library, Chembiochem: a European    journal of chemical biology 7, 834-838.-   Seidah, N., and Prat, A. (2002) Precursor convertases in the    secretory pathway, cytosol and extracellular milieu, Essays in    biochemistry 38, 79-94.-   Sellamuthu et al., Biochem. Biophys. Res. Commun., 371(1):122-126,    2008.-   Sellamuthu et al., PLoS One, 6(7):e22554, 2011.-   Semenza et al., Cell, 61(7):1349-1357, 1990.-   Sinha, J., Plantz, B., Inan, M., and Meagher, M. (2005) Causes of    proteolytic degradation of secreted recombinant proteins produced in    methylotrophic yeast Pichia pastoris: case study with recombinant    ovine interferon-tau, Biotechnology and bioengineering 89, 102-112.-   Small et al., Proteomics, 4(6):1581-90, 2004.-   Sudbery, P. (1996) The expression of recombinant proteins in yeasts,    Current opinion in biotechnology 7, 517-524.-   Teasdale and Jackson, Cell Dev. Biol. 12, 27-54, 1996.-   Tropea et al., Methods Mol. Biol., 498:297-307, 2009.-   Varadarajan et al., Angew. Chem. Int. Ed. Engl., 47(41):7861-7863,    2008.-   Varadarajan et al., J. Am. Chem. Soc., 131(50):18186-18190, 2009a.-   Varadarajan et al., Nat. Chem. Biol., 4(5):290-294, 2008.-   Varadarajan et al., Nat. Protoc., 4(6):893-901, 2009b.-   Varadarajan et al., Proc. Natl. Acad. Sci. USA, 102(19):6855-6860,    2005.-   Varkaris et al., Cancer Metastasis Rev. 33(2-3):595-606, 2014.-   Villa et al., J. Biol. Chem., 278(43):42545-42550, 2003.-   Watanabe et al., J Biosci Bioeng. 89(6):569-76, 2000.-   Waugh, Protein Expr. Purif., 80:283-293, 2011.-   Wehr et al., Nat. Methods, 3:985-993, 2006.-   Yi et al. “Engineering of TEV protease variants by yeast ER    sequestration screening (YESS) of combinatorial libraries.” Proc    Natl Acad Sci USA. 110(18):7229-34, 2013.-   Yi et al., (2015) Methods Mol Biol. 1319:81-93.-   Yi, L., Gebhard, M., Li, Q., Taft, J., Georgiou, G., and    Iverson, B. (2013) Engineering of TEV protease variants by yeast ER    sequestration screening (YESS) of combinatorial libraries,    Proceedings of the National Academy of Sciences of the United States    of America 110, 7229-7234.-   Zhou, A., Webb, G., Zhu, X., and Steiner, D. (1999) Proteolytic    processing in the secretory pathway, The Journal of biological    chemistry 274, 20745-20748.

1-47. (canceled)
 48. An isolated mutant ABL kinase, wherein the mutantABL kinase comprises a substitution mutation selected from the groupconsisting of Table 3; with amino acid position numbering beingaccording to the Kabat system.
 49. The isolated mutant ABL kinase,wherein the substitution mutation is V488L.
 50. The isolated mutant ABLkinase, wherein the substitution mutation is F493L or D445G.
 51. Anucleic acid encoding the mutant ABL kinase of claim
 48. 52. The nucleicacid of claim 51, wherein the mutant ABL kinase comprises thesubstitution mutation V488L.
 53. A host cell that recombinantlyexpresses the isolated mutant ABL kinase of claim
 48. 54. The host cellof claim 53, wherein the mutant ABL kinase comprises the substitutionmutation V488L.
 55. The host cell of claim 53, wherein the cell is aeukaryotic cell.
 56. The host cell of claim 55, wherein the eukaryoticcell is a yeast cell.